Devices such as a cellular phone, a digital camera, and a personal digital assistant are widely used and show a tendency of further prevalence. Hitherto, in those applications, a resin substrate, such as an acrylic resin substrate, has been used as a protective member for protecting a display. However, owing to its low Young's modulus, the acrylic resin substrate is liable to bend when a display surface of the display is pushed with a pen, a human finger, or the like. Therefore, the resin substrate causes a display failure through its contact with an internal display in some cases. The acrylic resin substrate also involves a problem of being liable to have flaws on its surfaces, resulting in easy reduction of visibility. A solution to those problems is to use a glass sheet as the protective member. The glass sheet (cover glass) is required to (1) have a high mechanical strength, (2) have a low density and a light weight, (3) be able to be supplied at low cost in a large amount, (4) be excellent in bubble quality, (5) have a high light transmittance in a visible region, and (6) have a high Young's modulus so as not to bend easily when its surface is pushed with a pen, a finger, or the like. In particular, a glass sheet that does not satisfy the requirement (1) cannot serve as the protective member, and hence a glass sheet tempered by ion exchange treatment or the like (so-called tempered glass sheet) has been used as the protective member heretofore.